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.TH HEAD "1P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
head
\(em copy the first part of files
.SH SYNOPSIS
.LP
.nf
head \fB[\fR-n \fInumber\fB] [\fIfile\fR...\fB]\fR
.fi
.SH DESCRIPTION
The
.IR head
utility shall copy its input files to the standard output, ending the
output for each file at a designated point.
.P
Copying shall end at the point in each input file indicated by the
.BR \-n
.IR number
option. The option-argument
.IR number
shall be counted in units of lines.
.SH OPTIONS
The
.IR head
utility shall conform to the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 12.2" ", " "Utility Syntax Guidelines".
.P
The following option shall be supported:
.IP "\fB\-n\ \fInumber\fR" 10
The first
.IR number
lines of each input file shall be copied to standard output. The
application shall ensure that the
.IR number
option-argument is a positive decimal integer.
.P
When a file contains less than
.IR number
lines, it shall be copied to standard output in its entirety. This
shall not be an error.
.P
If no options are specified,
.IR head
shall act as if
.BR "\-n 10"
had been specified.
.SH OPERANDS
The following operand shall be supported:
.IP "\fIfile\fR" 10
A pathname of an input file. If no
.IR file
operands are specified, the standard input shall be used.
.SH STDIN
The standard input shall be used if no
.IR file
operands are specified, and shall be used if a
.IR file
operand is
.BR '\-' 
and the implementation treats the
.BR '\-' 
as meaning standard input.
Otherwise, the standard input shall not be used.
See the INPUT FILES section.
.SH "INPUT FILES"
Input files shall be text files, but the line length is not restricted
to
{LINE_MAX}
bytes.
.SH "ENVIRONMENT VARIABLES"
The following environment variables shall affect the execution of
.IR head :
.IP "\fILANG\fP" 10
Provide a default value for the internationalization variables that are
unset or null. (See the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 8.2" ", " "Internationalization Variables"
for the precedence of internationalization variables used to determine
the values of locale categories.)
.IP "\fILC_ALL\fP" 10
If set to a non-empty string value, override the values of all the
other internationalization variables.
.IP "\fILC_CTYPE\fP" 10
Determine the locale for the interpretation of sequences of bytes of
text data as characters (for example, single-byte as opposed to
multi-byte characters in arguments and input files).
.IP "\fILC_MESSAGES\fP" 10
.br
Determine the locale that should be used to affect the format and
contents of diagnostic messages written to standard error.
.IP "\fINLSPATH\fP" 10
Determine the location of message catalogs for the processing of
.IR LC_MESSAGES .
.SH "ASYNCHRONOUS EVENTS"
Default.
.SH STDOUT
The standard output shall contain designated portions of the input
files.
.P
If multiple
.IR file
operands are specified,
.IR head
shall precede the output for each with the header:
.sp
.RS 4
.nf

"\en==> %s <==\en", <\fIpathname\fR>
.fi
.P
.RE
.P
except that the first header written shall not include the initial
<newline>.
.SH STDERR
The standard error shall be used only for diagnostic messages.
.SH "OUTPUT FILES"
None.
.SH "EXTENDED DESCRIPTION"
None.
.SH "EXIT STATUS"
The following exit values shall be returned:
.IP "\00" 6
Successful completion.
.IP >0 6
An error occurred.
.SH "CONSEQUENCES OF ERRORS"
Default.
.LP
.IR "The following sections are informative."
.SH "APPLICATION USAGE"
When using
.IR head
to process pathnames, it is recommended that LC_ALL, or at least
LC_CTYPE and LC_COLLATE, are set to POSIX or C in the environment,
since pathnames can contain byte sequences that do not form valid
characters in some locales, in which case the utility's behavior would
be undefined. In the POSIX locale each byte is a valid single-byte
character, and therefore this problem is avoided.
.SH EXAMPLES
To write the first ten lines of all files (except those with a leading
period) in the directory:
.sp
.RS 4
.nf

head -- *
.fi
.P
.RE
.SH RATIONALE
Although it is possible to simulate
.IR head
with
.IR sed
10q for a single file, the standard developers decided that the
popularity of
.IR head
on historical BSD systems warranted its inclusion alongside
.IR tail .
.P
POSIX.1\(hy2008 version of
.IR head
follows the Utility Syntax Guidelines. The
.BR \-n
option was added to this new interface so that
.IR head
and
.IR tail
would be more logically related. Earlier versions of this standard
allowed a
.BR \-number
option. This form is no longer specified by POSIX.1\(hy2008 but may
be present in some implementations.
.P
There is no
.BR \-c
option (as there is in
.IR tail )
because it is not historical practice and because other utilities in
\&this volume of POSIX.1\(hy2017 provide similar functionality.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIsed\fR\^",
.IR "\fItail\fR\^"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 8" ", " "Environment Variables",
.IR "Section 12.2" ", " "Utility Syntax Guidelines"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
